Fixed compile error on Mac: default arguments for lambda expressions

Summary:
Fixed the following compile error on Mac.

db/db_test.cc:8618:52: error: C++11 forbids default arguments for lambda expressions [-Werror,-Wlambda-extensions]
  auto gen_l0_kb = [this](int start, int size, int stride = 1) {
                                                   ^        ~
1 error generated.

Test Plan:
db_test